Method for production of ductile iron

ABSTRACT

The present invention relates to a method for nodularizing treatment of ductile iron using a ladle treatment method for nodularizing of a magnesium ferrosilicon alloy, where a magnesium ferrosilicon nodularizing alloy consisting essentially of 40 to 80% by weight of silicon, 2 to 15% by weight of magnesium, 0.3 to 5% by weight of lanthanum, 0 to 6% by weight of calcium, 0 to 5% by weight of aluminum, the balance being iron, is added to the ladle whereafter the molten iron is supplied to the ladle.

FIELD OF INVENTION

[0001] The present invention relates to a method for the manufacture ofductile cast iron with spheroidal or compacted graphite and aferrosilicon based nodularizer alloy.

BACKGROUND ART

[0002] Cast iron is typically produced in cupola or induction furnaces,and generally contains between 2 to 4 percent carbon. The carbon isintimately mixed with the iron and the form, which the carbon takes inthe solidified cast iron, is very important to the characteristics andproperties of the iron castings. If the carbon upon solidification takesthe form of iron carbide, then the cast iron is referred to as white orcarbidic cast iron and has the physical characteristics of being hardand brittle which in certain applications are undesirable. If the carbontakes the form of flake-like graphite, the cast iron is soft andmachinable and is referred to as grey cast iron. If magnesium and rareearth's are used to treat the liquid grey iron, the carbon will take theform of spheroidal or nodular graphite and is referred to as ductilecast iron.

[0003] The form, size and shape the graphite takes as well as the amountof graphite versus iron carbide, can be controlled with certainadditives that promote the formation of graphite during solidificationof cast iron. These additives are referred to as nodularizers andinoculants and their addition to the cast iron as nodularizing andinoculation. In casting iron products from liquid cast iron, there willalways be a risk for the formation of iron carbides in thin sections ofcastings. The formation of iron carbide is brought about by the rapidcooling of the thin sections as compared to the slower cooling of thethicker sections of the casting. The formation of iron carbide in a castiron product is referred to as “chill”. The formation of chill isquantified by measuring “chill dept” and the power of a nodularizer orinoculant to prevent chill and reduce chill depth is a convenient way inwhich to measure and compare the power of nodularizers and inoculants.

[0004] In cast iron containing spheroidal graphite the power ofnodularizers and inoculants is also commonly measured by the numberdensity per unit area of spheroidal graphite particles in the as-castmicrostructure. A higher number density per unit area of graphitespheroids means that the power of nodularizing and inoculation has beenimproved.

[0005] There is a constant need to develop nodularizers and inoculantswhich reduce chill depth and improve the machinability of ductile castirons as well as increase the number density of graphite spheroids.

[0006] Since the exact chemistry and mechanism of nucleation and whynodularizers and inoculants function as they do is not completelyunderstood, a great deal of research goes into providing the industrywith new and improved such alloys.

[0007] It is thought that rare earth's, such as cerium, lanthanum,praseodymium, and neodymium and certain other elements suppress theformation of iron carbide and promote the formation of graphite. Amajority of nodularizers contains rare earth's in the form of a mixturebetween cerium, lanthanum, praseodymium and neodymium, often knows asmisch metal. The addition of these elements is usually facilitated bythe addition of a magnesium ferrosilicon alloy and the most widely usedalloys contain 40 to 50% silicon, 4 to 6% magnesium and 1 to 2% mischmetal.

[0008] The suppression of carbide formation is associated by thenucleating properties of the nodularizer and inoculant. By nucleatingproperties it is understood the number of nuclei formed by an alloyaddition. A high number of nuclei formed improves the effectiveness andimproves the carbide suppression. Further a high nucleation rate mayalso give better resistance to fading effects during prolonged holdingtime of the molten iron after nodularizing and inoculation.

[0009] The nodularizer and inoculant alloys also affect ductile ironsolidification shrinkage. Some alloys may give good protection againstshrinkage while others tend to promote more shrinkage. The use ofvarious rare earth elements may have a pronounced impact on thiscondition. For nodularizer alloys it is also important that compositionof the alloy is such so that a minimum of shrinkage occurs duringsolidification of the iron.

[0010] The nodularizing process is carried out in two basicallydifferent ways. In the so-called “ladle treatment method” thenodularizer alloy is placed in the bottom of the ladle whereafter liquidcast iron is filled into the ladle on the top of the nodularizer alloy.Depending on how the nodularizer alloy is placed in the ladle, the ladletreatment method is known as overpour, sandwich, or tundish covertreatment methods. The inoculation is normally carried out after thenodularizing process is done, by adding inoculant to the metal streamduring transfer of the cast iron to a pouring vessel or to a mould.

[0011] In the so-called “in-the-mould” method the nodularizing treatmentis taking place inside the casting mould cavity itself. The in-the-mouldnodularizing method is thus significantly different from the ladletreatment nodularizing method.

[0012] It is known that the addition of lanthanum rare earth inmagnesium ferrosilicon alloy has proven successful for the purpose ofminimizing chill and shrinkage in ductile iron when using thein-the-mould nodularizing method. In the in-the-mould treatment methodthe magnesium ferrosilicon alloy will act both as nodularizer andinoculant simultaneously integrated in the gating system of the castingmould. For magnesium treatment of cast iron in the ladle treatmentnodularizing method such integrated or combined nodularizing andinoculation is not known.

DISCLOSURE OF INVENTION

[0013] It has now been found that the use of pure lanthanum as the onlyrare earth source in the MgFeSi nodularizer alloy surprisingly furtherimproves the performance of the ductile iron ladle treatment methodcompared to such nodularizers containing cerium or misch metal. Thus thenumber of nuclei is substantially increased and the risk for chill andshrinkage formation in the ductile or compacted graphite iron isminimized.

[0014] The present invention thus relates to a method for nodularizingtreatment of ductile iron using a ladle treatment method forintroduction of a magnesium ferrosilicon alloy, which method ischaracterized in that a magnesium ferrosilicon alloy consistingessentially of 40 to 80% by weight of silicon, 2 to 15% by weight ofmagnesium, 0.3 to 5% by weight of lanthanum, 0 to 6% by weight ofcalcium, 0 to 5% by weight of aluminum, the balance being iron, is addedto the ladle where after the molten iron is supplied to the ladle.

[0015] According to a preferred embodiment the magnesium ferrosiliconalloy added to the ladle comprises 0.5 to 1.5% by weight of lanthanum.

[0016] The present invention further relates to the use of a magnesiumferrosilicon alloy comprising 40 to 80% by weight of silicon. 2 to 15%by weight of magnesium 0.3 to 5% by weight of lanthanum, 0 to 6% byweight of calcium 0 to 5% by weight of aluminum, the balance being iron,as nodularizer in the ladle treatment method for the production ofductile cast iron.

[0017] It has surprisingly been found that when a magnesium ferrosiliconalloy is used in the ladle treatment method according to the presentinvention, the number of nuclei formed when the magnesium ferrosiliconalloy is added to cast iron, is increased thus obtaining higher nodulecounts and an improved suppression of iron carbide formation using thesame amount of alloy as with conventional ladle treatment methods andalloys.

[0018] Further, by the present invention it has been found that theshrinkage tendency of the ductile cast iron is greatly reduced or eveneliminated when using the method according to the present invention. Ithas also even been found that the present invention may providesufficient nucleation power to the ductile cast iron to avoid chill andshrinkage formations only with a very small, or even without, inoculantaddition following the nodularizing treatment process.

SHORT DESCRIPTION OF THE FIGURES

[0019]FIG. 1 is a schematic view of ladle treatment method used inexample 1,

[0020]FIG. 2 a-h show microstructures of test castings, and,

[0021]FIG. 3 a-d show shrinkage porosity in test castings.

DETAILED DESCRIPTION OF THE INVENTION Example 1

[0022] Ductile irons were produced in an induction furnace using acharge based on 50% by weight of steel, 20% by weight of iron returnsand 30% pig iron. The target analysis for the castings was 3.7% byweight of C, 2.4% by weight of Si, 0.4% by weight of Mn, 0.010% byweight of S and 0.040% by weight of Mg. Prior to tapping into anodularizing treatment ladle, 1.5% by weight of magnesium ferrosiliconalloy (MgFeSi) based on the weight of the cast iron was placed into theladle and covered by 0.5 kg steel punchings, i.e. nodularizing accordingto the sandwich treatment method. FIG. 1 shows a schematicrepresentation of the treatment ladle used. Two minutes after the ironwas tapped into the ladle, the iron was transferred into pouring ladles.Thus no inoculation was carried out after the nodularizating treatment.Coin shaped samples for chemical composition were extracted from themelt, and the ductile iron heats were then cast into sand moulds toproduce a 20 mm thick plate and a 5 mm thin plate, a standard chillwedge sample and a cross bar sample for shrinkage evaluation.

[0023] Four different tests were performed. In two of the testsmagnesium ferrosilicon alloy according to the present invention wereused, and for comparison purpose, one test was done with a rare earthfree magnesium ferrosilicon alloy and one test was run with a magnesiumferrosilicon alloy containing 1.0% by weight misch metal.

[0024] Table 1 shows the chemical composition of magnesium ferrosiliconalloys compared in this test, and Table 2 gives the chemical compositionof the produced ductile iron castings. TABLE 1 Chemical composition ofmagnesium ferrosilicon nodularizer alloys. Nodularizer % Si % Mg % Ca %Al % TRE % Ce % La 0.5% La 45.0 5.8 1.0 0.9 0.5 0.0 0.5 Invention 1.0%La 45.5 6.0 1.0 0.9 1.0 0.0 1.0 Invention RE-free 45.8 6.1 1.0 0.9 0.00.0 0.0 Prior art 1.0% Misch 45.0 5.9 1.1 0.8 1.0 0.5 0.25 Prior art

[0025] TABLE 2 Chemical composition of produced ductile iron castings.Ductile % % % % % % % % iron C Si Mn P S Mg Ce La 0.5% La 3.75 2.28 0.430.020 0.008 0.043 <0.004 0.008 1.0% La 3.73 2.25 0.42 0.024 0.010 0.040<0.004 0.015 RE-free 3.73 2.51 0.46 0.027 0.009 0.046 <0.004 <0.004 1.0%3.74 2.37 0.45 0.021 0.008 0.047 0.010 0.005 Misch

[0026] Results from metallographic evaluation of graphite structures incast plates samples are shown in Table 3. Microstructures of the testcastings are shown in FIG. 2, while Table 4 gives an evaluation of thechill condition of the microstructures of the test castings. Table 5 andFIG. 3 show results from measurement of shrinkage porosity in theexperimental cross-bar castings. TABLE 3 Characteristic graphite datafor cast 5 and 20 mm plates. Nodule Average Average Nodularizer countNodularity diameter shape alloy (N/mm²) (%) (μm) factor 5 mm plates 0.5%La 595 93 13.4 0.88 1.0% La 488 93 13.2 0.88 RE-free 110 81 9.3 0.751.0% MM 418 93 14.8 0.86 20 mm plates 0.5% La 224 78 17.5 0.74 1.0% La188 69 19.1 0.67 RE-free 112 42 20.2 0.50 1.0% MM 178 69 20.8 0.67

[0027] TABLE 4 Evaluation of chill condition in micrographs of FIG. 2.Nodularizer 5 mm plates 20 mm plates 0.5% La (e) no chill (f) no chill1.0% La (g) only trade of chill (h) no chill RE-free (a) massive chill(b) some chill 1.0% Misch (c) some chill (d) no chill

[0028] From Table 3 it can be seen that a substantial increase in thenumber of nodules was found in the two test runs according to theinvention compared to the method using rare earth free magnesiumferrosilicon alloy and to the method using magnesium ferrosilicon alloycontaining misch metal. This is true both for the 5 mm plates and the 20mm plates.

[0029] From FIG. 2 and Table 4 it can be seen that the lanthanumcontaining magnesium ferrosilicon alloys strongly reduces and nearlyeliminates the chill in the 5 mm plates and that no chill can be foundin the 20 mm plates.

[0030] Table 5 and FIG. 3 shows that shrinkage of the ductile iron iseliminated when using the method of the present invention.

[0031] From the Example it is clear that ductile iron having practicallyno chill can be produced by the present invention without carrying outthe conventional inoculation after the nodularizing treatment. Furtherthe method according to the invention results in a ductile cast ironhaving no shrinkage porosity. TABLE 5 Relative shrinkage porosity areain cross-bar castings. Nodularizer Area % pores 0.5% La 0.0 Invention1.0% La 0.0 Invention RE-free 8.3 Prior art 1.0% Misch 38.2 Prior art

[0032] Because of the low chill and shrinkage porosity formationtendency, especially for the 0.5% lanthanum containing magnesiumferrosilicon alloy, the need for a subsequent addition of post inoculantmaterial is minimized or even eliminated. Thus, the present inventiondescribes a unique new ladle treatment nodularizing method that will becost effective also in the sense that a minimum requirement forinoculation is needed.

1. Method for nodularizing treatment of ductile iron using a ladletreatment method for introduction of a magnesium ferrosilicon alloy,characterized in that a magnesium ferrosilicon nodularizing alloyconsisting essentially of 40 to 80% by weight of silicon, 2 to 15% byweight of magnesium, 0.3 to 5% by weight of tanthanum, 0 to 6% by weightof calcium, 0 to 5% by weight of aluminum, the balance being iron, isadded to the ladle whereafter the molten iron is supplied to the ladle.2. Method according to claim 1, characterized in that the magnesiumferrosilicon alloy added to the ladle comprises 0.5 to 1.5% by weight oflanthanum.
 3. Use of a magnesium ferrosilicon alloy comprising 40 to 80%by weight of silicon, 2 to 15% by weight of magnesium, 0.3 to 5% byweight of lanthanum, 0 to 6% by weight of calcium, 0 to 5% by weight ofaluminum, the balance being iron, as nodularizer in the ladle treatmentmethod for the production of ductile cast iron.